I think that Dallas can be considered "gallant" in some distinct ways. Dallas does believe in sacrificing for the Greasers. He believes in this larger conception of the good. This can be seen as "gallant" in that it embodies a sense of heroism to be so self- sacrificing. Dallas does not possess any duplicity towards the gang. He cares for them as he would a family. This becomes the source of his heroism or "gallantry."
Such sacrifice can be seen in how he looks out Johnny and Pony. He gains nothing from it. Yet, Dally understands that there is something larger than his own interests. It comes at great sacrifice to him in watching over the two boys in trouble. As he does this, one sees his own sense of gallantry in action. It is for this reason that Johnny sees Dallas as heroic as the leading men in Gone With the Wind. The idea that Dallas would be compared with a work like that is reflective of how gallant he is. Dallas is on the level with the heroes in Mitchell's work who sacrifice for something larger than themselves and their own interests. In this light, Dally can be seen as gallant.
Dallas Winston be considered gallant as in he literally give ponyboy the coat off his back when they desperately needed help and guidance when Johnny and Ponyboy after jonny accidently panicked and killed a soc.